As semiconductor feature sizes continue to shrink into the sub-micron range, the effects of light diffraction during photolithographic processes become more pronounced. Deep ultraviolet (UV) exposure tools use light sources having a 248 nm wavelength. When such tools are used to form semiconductor devices having feature sizes of 200 to 300 nm, the effects of light diffraction become quite pronounced.
One area where diffraction effects are a particular problem is in the formation of small rectangular features, such as contact holes and vias. Square features on a photomask pattern become rounded due to diffraction when the substrate wafer is exposed. Rounded holes have a greater resistance than rectangular contact holes of similar size, resulting in greater contact resistance. In order to prevent the increased resistance, a larger diameter round contact hole would be required, resulting in lower packing density (i.e. a greater spacing requirement). In either event, the rounded contact hole is less desirable than a rectangular contact hole of similar size.
The use of attenuated phase shift masks is well-known to offset the problems associated with light diffraction in small feature size products. The prior art offers many solutions which are geared toward eliminating side lobe patterns resulting from diffraction effects. For instance, Tzu teaches the use of an opaque ring around the phase shifting features to reduce the effects of side lobe formation in U.S. Pat. No. 5,935,736. Choi et al. teach the formation of “dummy” open regions in the phase shift mask, which dummy regions allow light to pass through unimpeded and 180 degrees out of phase with diffracted light that would otherwise form side lobes below the dummy open region in U.S. Pat. No. 5,591,550. Garza, in U.S. Pat. No. 5,795,682, also teaches the use of guard rings to destructively interfere with, and hence eliminate, side lobe patterns. Sugawara teaches the use of alternating patters of phase shifting and non-phase shifting patterns to cause destructive interference of side lobe patterns in U.S. Pat. No. 5,487,963.
The shortcoming of the prior art is that the various techniques to reduce or eliminate side lobe formation do not address the need for forming good rectangular patterns in dense patterns of small features. The present invention overcomes the shortcoming of the prior art, as will become apparent from the following description of preferred embodiments of the invention.